U.S. patent application number 11/985254 was filed with the patent office on 2008-06-19 for acceleration sensor.
Invention is credited to Fouad Bennini, Johannes Classen, Markus Heitz, Lars Tebje.
Application Number | 20080141774 11/985254 |
Document ID | / |
Family ID | 39277635 |
Filed Date | 2008-06-19 |
United States Patent
Application |
20080141774 |
Kind Code |
A1 |
Classen; Johannes ; et
al. |
June 19, 2008 |
Acceleration sensor
Abstract
An acceleration sensor includes a seismic mass which is
suspended on springs above a substrate and is deflectable in a
direction perpendicular to a surface of the substrate. In order to
reduce deflections of the seismic mass along the surface of the
substrate because of interference accelerations, which lead to a
falsification of the measurements of the deflection of the seismic
mass perpendicular to the surface of the substrate, the springs
include two bending bars which are interconnected via
crosspieces.
Inventors: |
Classen; Johannes;
(Reutlingen, DE) ; Heitz; Markus; (Kusterdingen,
DE) ; Tebje; Lars; (Reutlingen, DE) ; Bennini;
Fouad; (Reutlingen, DE) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
39277635 |
Appl. No.: |
11/985254 |
Filed: |
November 13, 2007 |
Current U.S.
Class: |
73/514.32 ;
73/514.38 |
Current CPC
Class: |
G01P 15/0802 20130101;
G01P 15/125 20130101 |
Class at
Publication: |
73/514.32 ;
73/514.38 |
International
Class: |
G01P 15/125 20060101
G01P015/125 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 13, 2006 |
DE |
102006053290.2 |
Claims
1. An acceleration sensor comprising: a substrate having a surface;
springs, each of the springs including two bending bars that are
interconnected via crosspieces; and a seismic mass suspended on the
springs above the substrate, the seismic mass being deflectable in
a direction perpendicular to the surface of the substrate.
2. The acceleration sensor according to claim 1, wherein each of
the two bending bars has a constant width.
3. The acceleration sensor according to claim 2, wherein the
crosspieces have a constant length, so that the two bending bars
run parallel to each other.
4. The acceleration sensor according to claim 1, wherein the
crosspieces are set apart uniformly between the two bending
bars.
5. The acceleration sensor according to claim 1, wherein the
springs are formed in one piece with the seismic mass.
6. The acceleration sensor according to claim 1, wherein each of
the springs has a bend.
7. The acceleration sensor according to claim 1, further comprising
two projections joining the springs to the seismic mass.
Description
BACKGROUND INFORMATION
[0001] An acceleration sensor includes a seismic mass which is
suspended on springs above a substrate, and is deflectable in a
direction perpendicular to a surface of the substrate.
[0002] The deflection of the seismic mass in a direction
perpendicular to the substrate because of an acceleration is
detected by an electrode provided below the seismic mass on the
substrate. Due to interference accelerations, the seismic mass is
able to be deflected not only in a direction perpendicular to the
surface of the substrate, but also along the surface of the
substrate. Because of inaccuracies when manufacturing the springs,
an interference acceleration along the surface of the substrate can
also lead to a deflection perpendicular to the substrate surface,
the measurement of the acceleration perpendicular to the surface of
the substrate thereby being falsified. Especially because of
vibrations, the seismic mass of the acceleration sensor can be
induced to oscillate with a natural frequency along the surface of
the substrate, these oscillations then falsifying the measurement
of the acceleration perpendicular to the surface of the substrate.
In this context, the natural frequency of the oscillations along
the surface of the substrate is a function of the stiffness of the
springs along the surface of the substrate.
[0003] In each case, the springs are made of an elongated, flexible
element. The stiffness of the springs along the surface of the
substrate can be increased by enlarging the cross-section of the
flexible elements. However, the stiffness of the springs
perpendicular to the surface of the substrate thereby increases at
the same time. The sensitivity of the acceleration sensor to
deflections in the z-direction decreases correspondingly.
SUMMARY OF THE INVENTION
[0004] An object of the present invention is to provide a sensitive
acceleration sensor which is set up to measure accelerations in one
direction, and whose measurements are scarcely falsified by
interference accelerations perpendicular to the direction.
[0005] According to the present invention, each spring includes two
bending bars that are interconnected via crosspieces.
[0006] Advantageously, such a spring exhibits great stiffness in a
direction along the surface of the substrate.
[0007] In one preferred specific embodiment, each of the two
bending bars has a constant width.
[0008] In a further refinement of the preferred specific
embodiment, the crosspieces have a constant length, so that the two
bending bars run parallel to each other.
[0009] In a further preferred specific embodiment, the crosspieces
are set apart uniformly between the bending bars.
[0010] Advantageously, the stiffness of the springs may thereby be
maximized in a direction along the surface of the substrate,
accompanied by a relatively low stiffness along a direction
perpendicular to the surface of the substrate.
[0011] In a further preferred specific embodiment, the springs are
formed in one piece with the seismic mass.
[0012] Advantageously, the seismic mass may be produced from
polysilicon by semiconductor manufacturing processes.
[0013] In yet a further preferred specific embodiment, each of the
springs exhibits a curvature.
[0014] Advantageously, it is possible to form springs with
sufficiently low stiffness in one direction perpendicular to the
surface of the substrate, without substantially increasing the
dimensions of the acceleration sensor.
[0015] In still another preferred specific embodiment, in each case
the springs are joined to the seismic mass via two projections.
[0016] In this manner, the maximum forces acting on the springs may
advantageously be reduced at the connection to the seismic
mass.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows a view of an acceleration sensor having springs
that are not bent.
[0018] FIG. 2 shows a view of an acceleration sensor having springs
that are bent once.
[0019] FIG. 3 shows a section of one of the springs from FIG. 2,
bent once.
[0020] FIG. 4 shows a diagram of the stiffnesses of one of the
springs from FIG. 2, bent once, as a function of a crosspiece
length.
[0021] FIG. 5 shows a view of an acceleration sensor having springs
bent three times.
[0022] FIG. 6 shows a view of an acceleration sensor having springs
bent five times.
DETAILED DESCRIPTION
[0023] FIG. 1 shows a view of an acceleration sensor having springs
2 that are not bent. The acceleration sensor is made of a structure
having a constant thickness, which is disposed above a substrate
that runs in the plane of the paper. For example, the structure is
produced by depositing a polysilicon layer with constant thickness
on an oxide layer, that in turn is provided on a silicon substrate.
Cavities are formed in the oxide layer, so that bonds from the
polysilicon layer to the silicon substrate develop in these
cavities. The structure is defined in the polysilicon layer by
etching, and the oxide layer is removed in an etching process. In
so doing, the polysilicon layer remains joined to the silicon
substrate.
[0024] The structure includes a square seismic mass 1 and four
springs 2 of the same kind, each of which is joined on one side of
seismic mass 1 via, in each case, two projections 3 at one end to
seismic mass 1. Instead of a square shape, other forms may also be
used for the seismic mass. At their other ends 4, springs 2 are
joined to the underlying substrate. Springs 2 are disposed parallel
to the sides of seismic mass 1, to thereby make the acceleration
sensor as compact as possible. Square holes 5 are formed in seismic
mass 1, so that an etching agent is easily able to penetrate to the
oxide layer during fabrication, and seismic mass 1 is completely
released from the underlying substrate. Each spring 2 has two
parallel bending bars 6 that are interconnected via crosspieces 7
and run at the same distance above the substrate. The two
projections 3 are likewise formed as two parallel bending bars 8
interconnected by crosspieces 9. The number of springs is
arbitrary. In any case, however, three springs must be provided in
order to suppress oscillations along the substrate surface. For
each spring, it is also possible to use more than two bending bars
that are interconnected via crosspieces.
[0025] When seismic mass 1 is deflected in a z-direction
perpendicular to the substrate because of an acceleration, the two
parallel bending bars 6 of each spring 2 deform. Projections 3 also
deform with bending bars 6. By connecting springs 2 to seismic mass
1 via two projections 3, which themselves are flexible, the
maximally occurring force can be reduced at the connection of
elastic springs 2 to rigid seismic mass 1, so that a rupture of
springs 2 may be prevented. Below seismic mass 1, an electrode (not
shown) is formed which detects a deflection of seismic mass 1 by
measuring a change in capacitance with respect to seismic mass 1.
In this context, oscillations of seismic mass 1 in the xy-plane
invalidate these measurements and should therefore be suppressed.
These oscillations are first of all suppressed by providing springs
2 on all four sides of square seismic mass 1. However, the
amplitude of these oscillations and the natural frequencies of the
spring-mass system made up of seismic mass 1 and springs 2 are also
a function of the stiffness of springs 2 in the x-direction and
y-direction, respectively.
[0026] In the further figures, the same reference numerals as in
FIG. 1 are used for elements of the same kind, or only different
elements are denoted by reference numerals.
[0027] FIG. 2 shows a view of an acceleration sensor having springs
10 that are bent one time. The four springs 10 of the same kind
have a bend 11, which is formed as a right angle of the two bending
bars 12 and 13. A formation of the bend as a right angle is
particularly space-saving for a square seismic mass 1.
[0028] FIG. 3 shows a section of spring 11 from FIG. 2, bent one
time. Parameter b denotes the width of a bending bar, parameter l
denotes the crosspiece length, parameter a denotes the crosspiece
width, and parameter s denotes the distance between two
crosspieces. In FIG. 2, all parameters s, a, b and l are constant,
but parameters s and a in particular can also vary. The two bending
bars may also have a different width and their width can vary, so
that l likewise can vary.
[0029] FIG. 4 shows a diagram of the relative stiffnesses of one of
the springs, bent once, along the substrate surface in the
y-direction (ky/ky.sub.0) and perpendicular to the substrate
surface in the z-direction (kz/kz.sub.0) as a function of
crosspiece length l which was calculated numerically by the finite
element method, the further parameters s, a and b being constant.
In this case, ky denotes the spring constant in the y-direction for
an arbitrary crosspiece length, ky.sub.0 denotes the spring
constant in the y-direction for 1=0, kz denotes the spring constant
in the z-direction for an arbitrary crosspiece length, and kz.sub.0
denotes the spring constant in the z-direction for 1=0. The
stiffness along the substrate surface increases as the crosspiece
length increases, while the stiffness in the z-direction is
independent of the crosspiece length. In this context, the spring
for 1=0 takes the form of an elongated flexible element, as is
known from the related art. By using a suitable crosspiece length
l, the stiffness of spring 10 along the substrate surface is able
to be increased as desired, so that the amplitude of the
oscillations of seismic mass 1 can be reduced to below a desired
threshold value, and the natural frequencies of seismic mass 1 are
able to be raised to the extent that they virtually cannot be
excited.
[0030] FIG. 5 shows a view of an acceleration sensor having springs
14 bent three times (U springs), and FIG. 6 shows a view of an
acceleration sensor having springs 15 bent five times (S springs).
The number of bends is arbitrary.
* * * * *